Method of making a triple density silicon nitride article

ABSTRACT

A method of making a triple density article of silicon nitride is disclosed. A first element is formed by hot pressing silicon nitride particles. The general shape of a second element is formed by injection molding silicon metal particles and a binder and subsequently burning out the binder. The general shape of a third element is slip cast adjacent and in bonding relationship to at least a portion of the second element. The so combined second and third elements are nitrided in a nitriding operation. Facing surface areas of the first element and the third element are bonded together by applying heat to both elements and pressure to at least one of the elements while the other element is held in a fixed position.

United States Patent [191 Styhr et al.

[ METHOD OF MAKING A TRIPLE DENSITY SILICON NITRIDE ARTICLE [75] Inventors: Karsten H. Styhr, Farmington;

Andre Ezis, Grosse Ile; Michael U. Goodyear, Plymouth, all of Mich.

[73] Assignee: Ford Motor Company, Dearborn,

Mich.

[22] Filed: Dec. 20, 1973 [21] Appl. No.: 426,835

[52] US. Cl. 156/89; 117/D1G. 12; 156/155; 156/242; 264/56; 264/58; 264/63; 416/241 [51] Int. Cl C03b 29/00 [58] Field of Search 156/89, 155, 288, 311, 156/242; 264/56, 57, 58, 60, 62, 66, 328,

332; l17/DIG. 12, 169 A; 75/226, 221, 214; 416/229, 241, 241 B; 29/15 A, 8 R, 8 B, 8

568,458 1/1959 Canada 416/241 B OTHER PUBLICATIONS A Biasi-2500F Target for Vehicular Ceramic Turbine June 3, 1975 Gas Turbine World Oct.-Nov., 1972 Vol. 2, No. 4 pgs. 12-19.

2500F Target for 30-MW Ceramics Test Turbine Gas Turbine World Aug. 1972 Vol. 2, No. 3 pgs. 34-40.

Gill et al.-Self Bonded Silicon Nitride Refractories Journal-March 1962. pgs. 92-96.

Popper et al. The Prep., Props, and Struct. of Silicon Nitride-Trans. of Brit. Cer. Soc. Vol. 60, 1961. pgs. 603-623.

Primary Examiner-Charles E. Van Horn Assistant Examiner-F. Frisenda, Jr.

' Attorney, Agent, or FirmWilliam E. Johnson; Keith L. Zerschling [5 7] ABSTRACT A method of making a triple density article of silicon nitride is disclosed. A first element is formed by hot pressing silicon nitride particles. The general shape of a second element is formed by injection molding silicon metal particles and a binder and subsequently burning out the binder. The general shape of a third element is slip cast adjacent and in bonding relationship to at least a portion of the second element. The so combined second and third elements are nitrided in a nitriding operation. Pacing surface areas of the first element and the third element are bonded together by applying heat to both elements and pressure to at least one of the elements while the other element is held in a fixed position.

6 Claims, N0 Drawings METHOD OF MAKING A TRIPLE DENSITY SILICON NITRIDE ARTICLE This application results from development work carried on for the Department of the Army under Contract No. DAAG-46-7l-COI62.

BACKGROUND OF THE INVENTION Silicon nitride has a wide variety of uses based on its physical and chemical properties. These uses, to name a few, include thermocouple protection tubes, crucibles for foundry use, substrates for electronic applications and structural components for gas turbine engines.

Silicon nitride can be produced by a number of different processing techniques with each technique yielding a different final density. Each technique also has a definite restriction on the final shape which may be produced. Simple shapes of better than 98% theoretical density can be made by hot pressing silicon nitride powder to form the final article. Complex shapes generally cannot be manufactured by this processing technique.

Another processing technique for manufacturing fairly intricate shapes of silicon nitride is slip casting. A slip casting technique generally produces a final article having a density in the range from 80 to 85% of theoretical density. In the slip casting technique, silicon metal particles are cast into the desired shape. The silicon metal is then converted, in a nitriding operation, into silicon nitride.

As an alternate to the above two techniques, silicon nitride articles of complex shape, having densities of 70 to 75% of theoretical density are produced by an injection molding technique. In this technique, silicon metal particles and a thermoplastic are formed into a mixture. This mixture is injection molded to form the shape of the article. Subsequent operations include the heating of the article to burn out the thermoplastic and a nitriding operation to produce the final silicon nitride article.

It is generally impossible to fabricate a complete rotor for a gas turbine engine of hot pressed, silicon nitride material. The impossibility of manufacturing such a complete rotor by a hot pressing technique comes about because of the complex shape of: the rotor blades. The complex shape of such blades can be formed easily by an injection molding technique. It is generally impossible, however, to form a complete rotor by an injection molding technique or by a slip casting technique as the hub portion of the rotor formed by either of these two techniques cannot withstand the mechanical and the thermal stresses imposed on that portion of the rotor during use in an engine.

This invention teaches a method wherein the best characteristics of hot pressed, slip cast and injection molded silicon nitride materials are brought together to form a complex article of manufacture such as a rotor for a gas turbine engine. All of the structure is bonded together by strong and uniform bonds at junctions between the different materials.

SUMMARY OF THE INVENTION This invention is directed to a method of making an article of silicon nitride and, more particularly, to a method of making a triple density article of silicon nitride. The method has the following general steps.

A first element of silicon nitride is formed in a hot pressing operation. In this operation, substantially pure silicon nitride particles are compacted to at least about 98% of theoretical density. The compacting of the silicon nitride occurs by applying both heat and pressure to a mixture of silicon nitride particles and a densification aid.

A second element has its general shape formed by injection molding silicon metal particles and a carrier, such as a thermoplastic, into a mold. The so molded article is heated to burn out the carrier.

The general shape of a third element is formed in physical association with the second element by slip casting the third element in contact with at least a portion of the second element. The slip casting of the third element is accomplished from a slip containing silicon metal particles suspended in a vehicle.

The second and third elements are nitrided in a single operation so that both of the elements are converted substantially to pure silicon nitride. By nitriding the two elements in a single operation, the two elements are joined to one another by a strong and permanent chemical bond at the interfacial area of slip casting contact.

A surface area on the first element is brought into close association with a surface area on the third element. The two areas are held in close association with one another. A bond is formed between the associated surfaces of the first and the third elements by applying heat to both elements and pressure on at least one of the elements while the other of the elements is held in a fixed position. This application of heat and pressure forces a portion of the silicon nitride forming the surface area of the one element into bonding relationship with the silicon nitride forming the surface area of the other element whereby a uniform and a strong bond is formed between the first and third elements thereby uniting all three of the elements into a single article having three sections of different density.

DESCRIPTION OF THE PREFERRED EMBODIMENT A process for making a triple density article of silicon nitride material will be described. The article to be described will be a rotor for a gas turbine engine. The first element of the rotor is its central hub. The second element of the rotor is the outer blade ring of complex shape over which the hot gases of the engine flow to turn the rotor. The third element is a transitional portion between the hub and the blade ring.

The method of making a triple density article of silicon nitride in accordance with the general teachings of this invention is initiated by forming a first element which, in this case, is the hub of the rotor. This first element is formed by compacting a mixture consisting of from about to 99.5% by weight of silicon nitride particles and from about 5 to 0.5% by weight of a densification aid. This mixture is compacted to a density of at least 98% theoretical density. The compacting of the mixture is carried out by applying heat and pressure to the mixture, thereby forcing it into a compact form.

In general, the silicon nitride use in the compacting operation is alpha silicon nitride powder. The powder is generally a ceramic grade and is preferably all minus 325 mesh. The silicon nitride powder is wet ball milled in a rubber lined mill with alumina or tungsten carbide balls and alcohol for a time ranging from 1 day to 2 weeks. A densification aid, such as magnesium oxide or any other suitable material. is mixed thoroughly with the silicon nitride powder during the milling operation. Concentrations of the densification aid are generally in the range from about 0.5 to 5.0 weight percent. The aid helps in the powder compacting process. After milling, the silicon nitride powder slurry is dried and screened through a 100 mesh screen for hot pressing.

The hub or first element is hot pressed to the approximate final dimensions and contours in a graphite die system. Barrier materials are coated on the graphite die to minimize any reaction between the powder and the die system. The barrier material commonly used is graphite foil. The silicon nitride material is hot pressed at a temperature from about 1,650C to about l,800C and at a pressure of from about 3,000 psi to about 4,000 psi. The pressing operation takes place for a time from 1 to 3 hours. The final article produced by this method is the first element of hot pressed silicon nitride material. The material has a density in excess of 98% theoretical and is extremely strong. The material readily withstands both the temperature and the stress load imposed when it is used as the hub portion ofa turbine rotor.

The general shape of the second element, which in this case is the turbine blade ring of complex configuration, is injection molded as a single unit. The blade ring has individual blades extending radially outwardly from a ring interconnecting all the blades. The interconnecting ring has an inwardly facing circumferential surface.

A typical molding compound for forming the second element is one in which the silicon metal particles comprise about 60 to 66% of its volume with the remainder being a thermoplastic binder. In general, the particle size of the metal will be such that the material will have a maximum particle size in the range of 40 to 60 microns and a mean particle size in the range from to 13 microns. Once a molding composition is formed, the composition is fed into a cylinder of an injection molding machine. The machine heats the thermoplastic above its melting point. Pressure is applied to the cylinder and the molding composition is shot into a cold molding die having the configuration of the blade ring to be produced. The thermoplastic solidifies into the desired shape thereby locking the silicon metal particles carried along with it into the desired shape.

The so formed blade ring of silicon metal and thermoplastic is then gradually heated in a furnace to a temperature of about 350C. The heating program may take as long as three days so that no stresses are created in the article during heating. During this heating, the thermoplastic binder is burned out. This action leaves behind silicon metal in the desired shape of the blade ring.

The general shape of the third element is formed in physical association with the shape of the second element by a slip casting technique. The third element is formed in contact with and facing inwardly from the inwardly facing circumferential surface of the second element, the blade ring. The third element is about 0.375 inches thick. In the slip casting technique, silicon metal particles carried by a suitable vehicle are deposited adjacent to the selected surface of the second ele ment and are built up. Full details of slip casting techniques are set forth in U.S. Patent Application Ser. No. 415,898, entitled Process for Making a Silicon Nitride Article" filed on Nov. 14, 1973 and assigned to the same assignee as this application. That application is hereby incorporated into this application byreference. In order to insure a very minimum of shrinkage in the slip cast portion of the article during nitriding. some coarser grain silicon metal or silicon nitride may be added to the casting slip or a slip having a solid content generally greater than 80% may be used.

In general, the slip casting technique consists of pouring a stable suspension of a powder and a vehicle, which together are called the slip, into an absorbent mold which represents the negative of the object being formed. The mold, which in this case includes a portion of the second element against which the slip is cast, by capillary action absorbs the liquid fraction of the slip leaving behind a coherent casting that has sufficient strength for handling. The full details of the particle size distribution and other characteristics of slip casting are set forth in the aforementioned patent application Ser. No. 415,898.

When the third element is formed by the slip casting technique adjacent the second element, the two elements are bonded together at the area of slip casting by a bond which is sufficiently strong to permit moderate handling of the two joined elements.

The joined elements are subjected to a nitriding operation. Since both elements are generally formed of pure silicon particles, the nitriding operation is effective to change both elements into silicon nitride. In the nitriding operation the elements are heated while exposed to nitrogen gas at temperatures and for sufficient periods of time that all of the silicon is transformed into silicon nitride.

A full procedure for nitriding silicon to form silicon nitride is disclosed in U.S. Patent Application Ser. No. 411,599, filed Oct. 30, 1973 entitled Improved Process for Making Silicon Nitride Articles, which application is assigned to the same assignee as this application. This application also is hereby incorporated by reference. In the nitriding operation, the injection molded silicon is converted into silicon nitride having a density of to of theoretical density whereas the slip cast portion of the silicon is transformed into a silicon nitride material having a density of about to of theoretical density. The bond formed between the second and the third elements in the nitriding operation is able to withstand the extreme temperatures and stresses that are normally encountered in the operation of a turbine engine.

The first element is bonded to the already joined second and third elements in the following manner. The first element, namely the hot pressed hub, has an outer circumferential surface which is brought into engagement with an inwardly facing circumferential surface formed on the third element of the now joined third and second elements. The surface areas which are brought into close association may be machined on each of the elements so that proper sizing is obtained. The associated elements are placed in a pressing die and heat and pressure are applied thereto while the joined second and third elements are supported to resist the pressure. The pressure applied on the central hub or first element is from at least about 1,000 lbs. per square inch to about 3,500 lbs. per square inch while the elements are maintained at a temperature in the range from 1.625C to about 1,700C for a time of at least 1.5 hours. During this hot pressing operation, a bond is formed between the associated surfaces of the first and third elements as the silicon nitride of both surfaces tends to migrate into the other surface thereby forming a uniform and strong bond therebetween.

The so completed article is thereby formed of silicon nitride material having three different densities. The central-portion of the formed article is hot pressed silicon nitride of high density. A transitional zone between the hub and the outer ring blade is formed of a slip cast silicon nitride material having a moderately high density. The complex turbine blades themselves are formed from injection molded material which produce a silicon nitride material of less than theoretical density. All three of the differently formed silicon nitride materials are uniformly bonded to one another to produce the final article which exhibits all the strength and temperature resistant characteristics for which silicon nitride is known.

There has been disclosed herein a method of making a triple density article of silicon nitride material. In view of the teachings of this specification, those skilled in the art will be led to make modifications of this in- "vention. It is intended that all modifications which fall within the spirit and scope of this invention be included within the appended claims.

We claim: 1. A method of making a triple density article of silicon nitride which comprises the steps of:

compacting a mixture consisting of from about 95 to 99.5% by weight silicon nitride particles and from about 5 to 0.5% by weight of a densification aid to at least 98% of theoretical density by use of heat and pressure thereby to form a first element of silicon nitride; forming the general shape of a second element by (a) injection molding silicon metal particles and a carrier to form the general shape of said second element, and (b) heating said second element to burn out said carrier; forming the general shape of a third element in physical association with the general shape of said second element by slip casting said third element adjacent to at least a portion of said second element,

6 said slip casting being accomplished from a slip containing silicon metal particles suspended in a vehicle;

nitriding said second and said third elements in a single operation so that both of said elements are converted substantially to pure silicon nitride and are joined to one another at the area of slip casting contact;

bringing a surface area of said first element into close association with a surface area of said third element;

holding said associated surfaces of said first and said third elements so they do not move with respect to one another; and

forming a bond between said associated surfaces of said first and said third elements by applying heat on all the elements and pressure on at least one of said elements while the other of said elements is held thereby to force a portion of the silicon nitride forming said surface area of said one element into bonding relationship with said surface area of the other element.

2. The method of claim 1 wherein: said first element is the one element to which pressure is applied to force its surface area into bonding relationship with said surface area of said third element.

3. The method of claim 1 wherein: said densification aid is magnesium oxide.

4. The method of claim 1 wherein: said compacting operation takes place at a temperature from of about 1,650C to about l,800C at a pressure of from at least 3000 psi and for a time of at least 1.5 hours.

5. The method of claim 1 wherein: said bond forming operation takes place at a temperature of from about 1,625C to about 1,700C at a pressure of at least 1,000 psi and for a time of at least 1.5 hours.

6. The method of claim 1 wherein: said suitable carrier used in the formation of said second element is a thermoplastic resin. 

1. A method of making a triple density article of silicon nitride which comprises the steps of: compacting a mixture consisting of from about 95 to 99.5% by weight silicon nitride particles and from about 5 to 0.5% by weight of a densification aid to at least 98% of theoretical density by use of heat and pressure thereby to form a first element of silicon nitride; forming the general shape of a second element by (a) injection molding silicon metal particles and a carrier to form the general shape of said second element, and (b) heating said second element to burn out said carrier; forming the general shape of a third element in physical association with the general shape of said second element by slip casting said third element adjacent to at least a portion of said second element, said slip casting being accomplished from a slip containing silicon metal particles suspended in a vehicle; nitriding said second and said third elements in a single operation so that both of said elements are converted substantially to pure silicon nitride and are joined to one another at the area of slip casting contact; bringing a surface area of said first element into close association with a surface area of said third element; holding said associated surfaces of said first and said third elements so they do not move with respect to one another; and forming a bond between said associated surfaces of said first and said third elements by applying heat on all the elements and pressure on at least one of said elements while the other of said elements is held thereby to force a portion of the silicon nitride forming said surface area of said one element into bonding relationship with said surface area of the other element.
 1. A METHOD OF MAKING A TRIPLE DENSITY ARTICLE OF SILICON NITRIDE WHICH COMPRISES THE STEPS OF: COMPACTING A MIXTURE CONSISTING OF FROM ABOUT 95 TO 99.5% BY WEIGHT SILICON NITRIDE PARTICLES AND FROM ABOUT 5 TO 0.5% BY WEIGHT OF A DENSIFICATION AID TO AT LEAST 98% OF THEORETICAL DENSITY BY USE OF HEAT AND PRESSURE THEREBY TO FROM A FIRST ELEMENT OF SILICON NITRIDE, FORMING THE GENERAL SHAPE OF A SECOND ELEMENT BY (A) INJECTION MOLDING SILICON METAL PARTICLES AND A CARRIER TO FORM THE GENERAL SHAPE OF SAID SECOND ELEMENT, AND (B) HEATING SAID SECOND ELEMENT TO BURN OUT SAID CARRIER, FORMING THE GENERAL SHAPE OF A THIRD ELEMENT IN PHYSICAL ASSOCIATION WITH THE GENERAL SHAPE OF SAID SECOND ELEMENT BY SLIP CASTING SAID THIRD ELEMENT ADJACENT TO AT LEAST A PORTION OF SAID SECOND ELEMENT, SAID SLIP CASTING BEING ACCOMPLISHED FROM A SLIP CONTAINING SILICON METAL PARTICLES SUSPENDED IN A VEHICLE, NITRIDING SAID SECOND AND SAID THIRD ELEMENTS IN A SINGLE OPERATION SO THAT BOTH OF SAID ELEMENTS ARE CONVERTED SUBSTANTIALLY TO PURE SILICON NITRIDE AND ARE JOINED TO ONE ANOTHER AT THE AREA OF SILIP CASTING CONTACT, BRINGING A SURFACE AREA OF SAID FIRST ELEMENT INTO CLOSE ASSOCIATION WITH A SURFACE AREA OF SAID THIRD ELEMENT, HOLDING SAID ASSOCIATED SURFACES OF SAID FIRST AND SAID THIRD ELEMENTS SO THEY DO NOT MOVE WITH RESPECT TO ONE ANOTHER, AND FORMING A BOND BETWEEN SAID ASSOCIATED SURFACES OF SAID FIRST AND SAID THIRD ELEMENT BY APPLYING HEAT ON ALL THE ELEMENTS AND PRESSURE ON AT LEAST ONE OF SAID ELEMENTS WHILE THE OTHER OF SAID ELEMENTS IS HELD THEREBY TO FORCE A PORTION OF THE SILICONE NITRIDE FORMING SAID SURFACE AREA OF SAID ONE ELEMENT INTO BONDING RELATIONSHIP WITH SAID SURFACE AREA OF THE OTHER ELEMENT.
 2. The method of claim 1 wherein: said first element is the one element to which pressure is applied to force its surface area into bonding relationship with said surface area of said third element.
 3. The method of claim 1 wherein: said densification aid is magnesium oxide.
 4. The method of claim 1 wherein: said compacting operation takes place at a temperature from of about 1,650*C to about 1, 800*C at a pressure of from at least 3000 psi and for a time of at least 1.5 hours.
 5. The method of claim 1 wherein: said bond forming operation takes place at a temperature of fRom about 1,625*C to about 1, 700*C at a pressure of at least 1,000 psi and for a time of at least 1.5 hours. 